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Most theoretical studies of seismic wave propagation in a porous medium do not predict a significant increase in SH-wave velocity with increasing water saturation. Although that type of behavior is commonly predicted for P-waves (and confirmed by countless observations), the expectation for SH-waves is a slight decrease in propagation velocity with increasing water saturations. While published measurements of SH-wave velocity in laboratory studies have been supportive of such a slight decrease in velocity, the data have been biased towards high pressures (typical of oil reservoirs at large depths of burial). On the other hand, the few published low pressure laboratory measurements have revealed significantly different results.

The authors’ in-situ measurements of seismic wave velocities in a shallow, coarse grained, unconfined alluvial aquifer document a significant SH-wave velocity increase in the transition from the vadose zone to the water table. In one vertical seismic profile (VSP), the P-wave velocity increases by a factor of 4.2 and the SHwave velocity increases by a factor of 2.6. What is not clear at this point is the true nature of the increase. Is the velocity increase an expression of the presence of water in the pores, or does water alter the rigidity of the soil matrix?

In addition to the broad-band velocity increase, we have also observed changes in the attenuation of SHwaves across the water table. After correcting for geometric spreading, the amplitude decay observed in the vadose zone has been found to be larger than that observed below the water table. However, the variation in amplitude decay as a function of frequency and the measurements of body wave dispersion were found to be larger below the water table than above. That is, the water saturated soil behavior is consistent with a Voigt solid, but the dry material appears to follow a different model.

The authors will discuss these observations in the context of the current debate on Poisson’s ratio and the Vp/Vs ratio.

Copyright Statement

This article was originally published by The Environmental and Engineering Geophysical Society (EEGS) in Symposium on the Application of Geophysics to Engineering and Environmental Problems 1996. Copyright restrictions may apply.